Photovoltaic devices can include semiconductor material deposited over a substrate using various production systems; for example, cadmium sulfide (CdS) or cadmium telluride (CdTe) thin films are deposited over a substrate using a VTD system. A typical VTD system may use a powder delivery unit, a powder vaporization and distribution unit, and a vacuum deposition unit. The powder delivery unit may include a vibratory powder feeder.
Vibratory powder feeders are designed to deliver raw material powder to the vaporizer unit. In conventional vibratory powder feeders, raw semiconductor material powder is loaded into the feeder. A controlled amount of carrier gas is directed into the feeder to facilitate the flow of powder. The powder and carrier gas then flow to the vaporizer unit. The carrier gas is typically inert and does not chemically react with the powder. The purpose of the carrier gas is to facilitate transport of the powder to the vaporizer unit.
FIG. 1 illustrates an example of a conventional vapor transport deposition system 11 for delivering and depositing a semiconductor material, for example, CdTe onto a substrate 13, for example, a glass substrate used in the manufacture of thin film solar modules. A vaporizer unit 15 has walls formed of a resistive material which can be heated by AC power source 29 and vaporizes CdTe semiconductor material powder received at opposite ends of vaporizer unit 15 through respective inlets 17 and 19 which receive the semiconductor material powder from respective vibratory powder feeders 21 and 23. Inert carrier gas sources 25 and 27, for example, Helium gas (He) sources, respectively provide a carrier gas to the powder feeders 21 and 23 to transport the semiconductor material through inlets 17 and 19 to the vaporizer unit which vaporizes the semiconductor material powder for deposition of the semiconductor material onto substrate 13. More detailed examples of vapor transport deposition systems of the type illustrated in FIG. 1 can be found in U.S. Pat. Nos. 5,945,165, 6,037,241, and 7,780,787, all assigned to First Solar, Inc.
The heating of the vaporizer unit 15 is not uniform along its length because the vaporizer unit 15 is formed of three separate sections of resistivity material, e.g., SiC, which are connected together along the length of the vaporizer unit 15. Thus, as shown in FIG. 2A, a hot zone is provided by section 31 throughout most of the length of vaporizer unit 15, while a lower temperature colder zone is provided at respective ends of vaporizer unit 15 by section 33. As a consequence, some of the vaporized semiconductor material condenses and accumulates in the colder zones as condensed semiconductor material 35. When the carrier gas source 25 or 27 is turned off during powder refills or for maintenance of one or both of the powder feeders 21 and 23, the built up condensed semiconductor material 35 can resublimate and back diffuse into the inlets 17 and 19 causing a clog, as illustrated in FIG. 2B. Unclogging the inlets 17 and 19 is time consuming and costly. In addition, when one or both of the carrier gas sources 25 and 27 are turned off, the partial pressures of reactive species, such as oxygen (O2) and vaporized water (H2O), within the vapor transport deposition system can change. When one or both of the carrier gas sources 25 and 27 are then turned on, the gas ambient for coating deposition is different from what it was before one or both of the carrier gas source 25 and 27 was turned off. This produces an unstable flow field and gas composition. When flow of the carrier gas source 25 or 27 resumes, the unstable flow field and gas composition causes deposition variations on substrate 13.
Furthermore, it may be desirable for certain depositions to introduce a dopant into the semiconductor material which can react with semiconductor material and form a vapor phase compound within vaporizer unit 15 during the deposition process. To provide for this doping, a process gas, such as feeder compressed dry air (O2), is also introduced into the vaporizer 15 to provide a reactive mix with the dopant. Introduction of the dopant and process gas into vaporizer unit 15 can cause formation of a gas phase product and a solid phase product. While the gas can pass through the porous walls of vaporizer unit 15 for deposition on a substrate 13 (FIG. 1), the solid can not and is confined within the vaporizer causing vaporizer pore clogging. Controlling the flow of process gas into the vaporizer 15 provides a mechanism to adjust doping concentration by controlling the dopant, process gas reaction. This doping concentration control is important for the manufacturing process. Accordingly, a controlled flow of reaction gas and dopant which favors formation of a gaseous product over a solid product needs to be established and maintained to carry out doping of semiconductor depositions and avoid vaporizer pore clogging.
An improved feeder system which reduces the impact of the noted problems is desirable.